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    <title>ShellCheck: SC2236 – Use `-n` instead of `! -z`.</title>
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    <h1>SC2236 – ShellCheck Wiki</h1>
    <a href="https://github.com/koalaman/shellcheck/wiki/SC2236">See this page on GitHub</a>
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    <h2 id="use--n-instead-of---z">Use <code>-n</code> instead of
<code>! -z</code>.</h2>
<p>(or "Use <code>-z</code> instead of <code>! -n</code>")</p>
<h3 id="problematic-code">Problematic code:</h3>
<div class="sourceCode" id="cb1"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb1-1"><a href="SC2236.html#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="cf">if</span> <span class="bu">[</span> <span class="ot">!</span> <span class="ot">-n</span> <span class="st">&quot;</span><span class="va">$JAVA_HOME</span><span class="st">&quot;</span> <span class="bu">]</span><span class="kw">;</span> <span class="cf">then</span> <span class="bu">echo</span> <span class="st">&quot;JAVA_HOME not specified&quot;</span><span class="kw">;</span> <span class="cf">fi</span></span>
<span id="cb1-2"><a href="SC2236.html#cb1-2" aria-hidden="true" tabindex="-1"></a><span class="cf">if</span> <span class="bu">[</span> <span class="ot">!</span> <span class="ot">-z</span> <span class="st">&quot;</span><span class="va">$STY</span><span class="st">&quot;</span> <span class="bu">]</span><span class="kw">;</span>       <span class="cf">then</span> <span class="bu">echo</span> <span class="st">&quot;You are already running screen&quot;</span><span class="kw">;</span> <span class="cf">fi</span></span></code></pre></div>
<h3 id="correct-code">Correct code:</h3>
<div class="sourceCode" id="cb2"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb2-1"><a href="SC2236.html#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="cf">if</span> <span class="bu">[</span> <span class="ot">-z</span> <span class="st">&quot;</span><span class="va">$JAVA_HOME</span><span class="st">&quot;</span> <span class="bu">]</span><span class="kw">;</span> <span class="cf">then</span> <span class="bu">echo</span> <span class="st">&quot;JAVA_HOME not specified&quot;</span><span class="kw">;</span> <span class="cf">fi</span></span>
<span id="cb2-2"><a href="SC2236.html#cb2-2" aria-hidden="true" tabindex="-1"></a><span class="cf">if</span> <span class="bu">[</span> <span class="ot">-n</span> <span class="st">&quot;</span><span class="va">$STY</span><span class="st">&quot;</span> <span class="bu">]</span><span class="kw">;</span>       <span class="cf">then</span> <span class="bu">echo</span> <span class="st">&quot;You are already running screen&quot;</span><span class="kw">;</span> <span class="cf">fi</span></span></code></pre></div>
<h3 id="rationale">Rationale:</h3>
<p>You have negated <code>test -z</code> or <code>test -n</code>,
resulting in a needless double-negative. You can just use the other
operator instead:</p>
<pre><code># Identical tests to verify that a value is assigned
[ ! -z foo ] # Not has no value
[ -n foo ]   # Has value

# Identical tests to verify that a value is empty
[ ! -n foo ] # Not is non-empty
[ -z foo ]   # Is empty</code></pre>
<h3 id="exceptions">Exceptions:</h3>
<p>This is a stylistic issue that does not affect correctness. If you
prefer the original expression, you can <a href="Ignore.html">Ignore</a> it
with a directive or flag.</p>
<h3 id="related-resources">Related resources:</h3>
<ul>
<li>Help by adding links to BashFAQ, StackOverflow, man pages, POSIX,
etc!</li>
<li><a
href="https://www.gnu.org/software/bash/manual/html_node/Bash-Conditional-Expressions.html#Bash-Conditional-Expressions">Bash
reference manual for -z and -n</a>.</li>
<li>Note: Be careful with quoting variables (which you should always do
anyway): <code>[ ! -z $var ]</code> might work, but
<code>[ -n $var]</code> will not. <code>[ -n "$var" ]</code> will do
what you expect.</li>
</ul>
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